The present invention relates to a circuit for emphasizing the aperture of an image in a video screen display system and in particular, to a circuit that compensates the level of the aperture component to add the compensated aperture component to the video signal.
The low transition level of an ordinary video signal can become deteriorated during transmission through a transmission media (cable/atmosphere), recording onto a record media or during reproduction. Correcting the aperture of a video signal to emphasize the outline of a picture before transmission and recording is done to prevent loss at low transition levels. Furthermore, an aperture correction circuit that corrects the aperture of a picture improves the picture resolution by separating the aperture component having level differences between the pixels from the video signal for adding to the original video signal. However, because the conventional aperture correction circuit does add the aperture component separated from the video signal to the original video signal as a separated aperture component that is not compensated when the level of the aperture component is low, the corrected video signal does not have a sufficient degree of resolution. An explanation of the problems with the conventional aperture correction circuit follows with reference to the attached illustrations.
FIG. 1 is a circuit diagram of a conventional vertical aperture correction circuit, FIG. 2 is a circuit diagram of a conventional horizontal aperture correction circuit, and FIGS. 3A through 3D show the output wave forms of the respective sections illustrated in FIGS. 1 and 2.
First, FIG. 1 will be explained with reference to FIG. 3. At input terminal 10, a luminance signal as shown in FIG. 3A is applied. Two dot broken-line surrounded section 34, composed of two line delays 12, 14, adder 16 and subtracter 18, is a differentiation circuit for detecting the aperture component as shown in FIG. 3B from the level transition section as in FIG. 3A. Low pass filter 20 removes high frequency noise signals included in the aperture component from the differentiation circuit and applies the aperture component, free of high frequency noise, to coring section 22. As shown in FIG. 3C, coring section 22 removes that aperture component within a predetermined level range of positive and negative (+,-) with O as a reference, from the overall aperture component from the low-pass filter 20 to supply the resultant signal to the gain regulator section 24. Gain regulator section 24 amplifies the aperture component from the coring section 22 to regulate the aperture component's gain.
Additionally, limiter 26 limits the amplified aperture component input from the gain regulator section 24 to a certain positive (+) and negative (-) level and applies the limited aperture component to adder 30. Delay 28 delays the luminance signal entered from the line delay 12 by the amount of propagation delay time in the circuits consisting of line delay 14 adder 16, subtracter 18, low pass filter 20, coring section 22, gain regulator section 24 and limiter 26 for output to adder 30. Adder 30 adds to the luminance signal from delay 28 the aperture component from limiter 26 and as in FIG. 3D, outputs an aperture corrected luminance signal through output terminal 32.
Secondly, FIG. 2 will be explained in detail with reference to FIG. 3. In FIG. 2, at input terminal 100, a luminance signal is applied as in FIG. 3A. Band pass filter 102 detects the aperture component as in FIG. 3B from the level transition section of the luminance signal as in FIG. 3A being applied to input terminal 100 and applies the detected aperture component to coring section 104. Here, the aperture component detected in the band pass filter 102 exists in the horizontal direction. Therefore, band pass filter 102 includes a low pass filter and a differentiation circuit where two pixel delays are inserted instead of two line delays 12, 14 as shown in the differential circuit of FIG. 1. As shown in FIG. 3C, coring section 104 removes the positive (+) and negative (-) aperture component existing within a certain level range with reference to a "O" level from the aperture components in the band pass filter 102 and applies resultant signal to gain regulator section 106. Gain regulation section 106 amplifies the aperture component from coring section 104 and inputs the amplified aperture component to limiter 108. Limiter 108 limits the amplified aperture component from the gain regulator section 106 to within a certain range level of plus (+) or (-) to supply the level limited aperture component to adder 112 Additionally, delay 110 causes a delay to the luminance signal applied at the input terminal 100 to the extent of the propagation delay time in the band pass filter 102, coring section 104, gain regulator section 106 and limiter 108 and supplies it to adder 112. Then, adder 112 adds the aperture component from limiter 108 to the luminance signal from delay 110 and as in FIG. 3D, transmits an aperture corrected luminance signal via output terminal 114.
As mentioned above, the conventional aperture correction circuit is not able to correct the aperture sufficiently when the transition level is low because it adds detected aperture component having a level corresponding to the transition level from the luminance signal to the original transition level from the luminance signal to the original luminance signal as a non-compensated condition. Therefore, the conventional aperture correction circuit is not able to correct the aperture of an video signal to have a sufficient degree of resolution.